The present invention relates to devices for centering tools in a cased well, and in particular for centering tools in deviated cased wells.
The work performed inside a well often requires that a tool which passes through the well be properly centered therein. This is particularly true when measuring the transverse dimensions of a well as described for example in French patent application No. 81 24021 filed Dec. 22, 1981, corresponding to U.S. Application No. 450,595 filed Dec. 17, 1982.
There are technical difficulties in developing entirely satisfactory centering devices. Firstly, the casing of a well has localised variations in diameter both at joints and at the valves situated at the well head. The centering device must therefore be capable of adapting to such variations in diameter. In the industry, such an adaptive centering device is often called a "centralizer". Furthermore, the recent tendency, particularly for offshore drilling, is to drill deviated wells which depart substantially from the vertical on one or more occasions. As a result, the operation of the centering device is perturbed by the effect of gravity on the tools centered by the device which effect operates laterally on the centering device.
Finally, to obtain proper centering, the device will by necessity bear against the walls of the well. In many prior art devices, sliding means are used to bear against the wall of the casing. This is true, in particular, for the centralizers described in U.S. Pat. Nos. 3,097,433, 3,555,689, and 3,915,229. This type of bearing is no longer entirely satisfactory, in particular in deviated wells, since it tends to alter the state of the inside wall of the casing, and thereby accelerate corrosion; furthermore it tends to limit the critical deviation angle beyond which the tool will no longer advance under the effect of its own weight.
In order to remedy the above drawbacks, a centralizer has been proposed in the literature in which contact with the wall of the well casing is provided by wheels, e.g., "Improved Technique for Logging High-Angle Well" by M. W. Bratovich, W. T. Bell & K. D. Kaaz, published under the reference SPE 6818 by the American Institute of mining Metallurgical Petroleum Engineers, Inc. In FIG. 3B of that article a centering device is shown which comprises a central bar suitable for being attached to a tubing string for lowering a tool in a cased well, two moving slides capable of sliding to a limited extent over the central bar, a series of radial wheel-carrying structures of generally triangular geometry defined by arms articulated to the two slides, a series of wheels mounted free to rotate on each of the radial structures in such a manner that the periphery of each series of wheels projects radially outwardly, and resilient return means which urge the wheels radially outwardly. Although this type of centralizer enables some work to be performed in a deviated well, it is not free from drawbacks.
Firstly, its centering or bearing force on the walls of the casing tends to fall off too rapidly when the diameter of the casing increases. Secondly, in order to satisfy maximum diameter requirements when in the retracted position, e.g., less than 50 mm, the wheels must be of small diameter; they therefore rotate very quickly while a tool with the device is going down a well, which considerably reduces their lifetime. Finally, this type of centralizer is subject to severe shocks each time it passes a joint in the tubing, which shocks rapidly puts the device out of service.